With the recent development of micro-machining technology for ultra-large scale integrated circuits (ULSI), a design rule of 20 nm is realized. CMP has been spotlighted as an important process for planarization of a photoresist layer, leading to improved precision of patterns formed after exposure of the planarized surface and thus increasing manufacturing yield of semiconductor devices. In particular, since shallow trench isolation (STI) is the first process in semiconductor processing to which the most precise design is applied, planarization after STI is a core process in device fabrication.
CMP slurries comprising ceria particles have attracted attention as a major material for controlling selectivity of a polishing speed between silicon nitride (Si3N4) deposited to a thickness of about 300 to about 500 Å as an STI pattern mask in GATE formation sites and silicon oxide (SiO2) layer deposited on a trench isolation region and silicon nitride.
In STI, oxides filling trenches for isolation of devices after deposition of a nitride layer mask at device formation sites may be over-deposited by up to 7,000 Å on the nitride layer so as to ensure complete filling.
At this point, a step of 2,000 Å to 3,000 Å may be formed between the oxide layer deposited on the nitride layer and the oxide layer deposited in a trench of about 2,000 to about 2,500 Å. Accordingly, STI CMP consists of three steps, i.e., primary polishing for removing the step between over-deposited oxide layer (convex portion) on the nitride layer and the oxide layer (concave portion) on the trench, secondary polishing for removing the oxide on the nitride layer, and tertiary polishing for over-polishing in order to completely remove remaining oxide on the nitride layer.
In primary polishing, the oxide step is rapidly removed to provide good productivity. In secondary polishing, a planarized surface is formed by polishing the oxide layer to a thickness of about 500 to about 1000 Å on the nitride layer in order to prevent the oxide layer in the trench from being polished, instead of the nitride layer, to the height or less of the nitride layer. In tertiary polishing, the nitride layer is subjected to over-polishing to a thickness of 100 Å or less in order to completely remove the oxide layer on the nitride layer while minimizing loss (dishing) of the oxide layer in trench.
Ceria abrasives have strong reactivity with an oxide layer and thus, at a low concentration of about 1% or less, the ceria abrasives may exert a polishing speed that is about two times faster than silica abrasives requiring a concentration of about 10% or more. Recently, ceria abrasives have been developed so as to prevent CMP defects by reducing a particle diameter to about 100 nm.
Therefore, there is an urgent need for a CMP slurry composition capable of maintaining a polishing speed of about 2000 Å/minute or more when polishing an oxide layer deposited on a nitride layer while maintaining a ratio of the polishing speed of the oxide layer on the trench to the polishing speed of the nitride layer at about 50 or more, while keeping surface defects of the overall layer below 70 nm in size upon polishing.